Republican presidential hopeful Scott Walker vowed Sunday that he would defeat Islamic terrorism by “unleashing” U.S. military power that has been hamstrung by the Obama administration, part of a series of moves by the Wisconsin governor to convince voters that he has the national security chops to be commander in chief.

Mr. Walker said he would lift “political restrictions” on U.S. forces already deployed to help fight terrorists in the Middle East that were imposed by President Obama and supported by Democratic presidential front-runner Hillary Rodham Clinton when she was secretary of state.

“We have people in Iraq right now in the military, over 3,000 troops. It’s not a question of sending more in. It’s about empowering them to unleash the power of the United States military,” Mr. Walker said on NBC’s “Meet the Press.”

“We have people right there as air controllers who could literally draw in airstrikes with absolute precision. They can’t do that,” he said. “I talked to a general earlier this year who said airstrikes can be effective, but right now, they’re like a drizzle. He said, we need to have a thunderstorm there,” Mr. Walker said.

The hawkish rhetoric built upon Mr. Walker’s speech last week at the Citadel in South Carolina, where he described the U.S. war with radical Islam as a “generational struggle” and pledged to defeat the Islamic State terrorist army and curb Iran’s influence in the Middle East.

Mr. Walker entered the race as a top contender. But like many other prominent Republican candidates this year, he was eclipsed by billionaire businessman Donald Trump and now finds himself clumped in the middle of a crowded field of candidates and struggling to stand out.

At the Citadel, Mr. Walker said he shares former President Ronald Reagan’s goal of achieving “peace through strength” and that confronting the Islamic State, also known as ISIS and ISIL, and preventing Iran from obtaining a nuclear bomb were critical to keeping the nation safe.

“But let me be clear: Defeating ISIS and rolling back Iran will require a greater investment of U.S. resources,” he said before taking a shot at the Obama administration. “Sternly worded tweets and isolated airstrikes will not destroy this enemy.”

He said the Islamic State will not be defeated as long as Syrian President Bashar Assad “is still in power and Iran, his patron, has a base of operations in Damascus.”

“Therefore, we must do more to recruit and support fighters in Syria who oppose both ISIS and Assad,” he said.

“The Obama administration has only trained about 60 fighters — that is not going to cut it. A no-fly zone enforced by America and its allies would dramatically enhance this campaign on the ground,” he said. “Our efforts in Syria and Iraq must be part of a broader, U.S.-led regional coalition, with real buy-in and ironclad guarantees from our allies that they will help us shoulder the burden.”

Polling shows that Mr. Walker’s star is fading in the Republican nomination race. He is now running sixth nationally and third in Iowa, which kicks off the race with its caucuses and is seen as crucial to his chances of capturing the nomination.

Mr. Walker has made the case that the leadership he showed in Wisconsin — including during his showdown with labor unions over collective bargaining rights — would carry over to his decision-making on the foreign policy front.

“With all of the challenges we face around the globe today, now is not the time for untested leadership. I have been tested like no other candidate in this race,” he said Friday.

On his first day in office, Mr. Walker said, he would terminate the nuclear deal that the U.S. — along with the United Kingdom, France, China, Russia and Germany — has agreed to with Iran that eases economic sanctions in exchange for limiting the scope of the Islamic republic’s nuclear program.

“Then, I will immediately reimpose sanctions against Iran and work with Congress to impose even more crippling sanctions,” he said.

He also rehashed familiar Republican attacks against Mr. Obama and former Secretary of State Hillary Rodham Clinton, blaming them for weakening the United States by “leading from behind” on global issues.

Mr. Walker said Libya is now a “safe haven” for terrorists and criticized the way Mrs. Clinton handled the attacks of diplomatic posts in Benghazi that led to the deaths of the U.S. ambassador to Libya and three other Americans.

“Everywhere in the world that Hillary Clinton has touched is more messed up now than before she and the president took office,” he said.

“In the real world — the world outside Washington — when you fail at one job you don’t get promoted to another,” he said, alluding to Mrs. Clinton, the front-runner for the Democratic presidential nomination.